Tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] was first introduced to the United States in the late 19th century but did not become widely used until the 1940s. Now it is one of the more significant cool-season forage grasses with more than 1 million acres planted in north Georgia alone. Tall fescue is the most heat tolerant of the cool-season grasses because of a fungus, called an “endophyte,” that grows within the plant. While the nutrient composition of tall fescue compares favorably with other cool-season grasses, the endophyte produces ergot alkaloids that can have negative effects on animals that eat the fungus-infected forage.
Microscopic image of Epicloe coenophialum by Nick Hill, 海角官方首页
The specific endophyte (E+) in tall fescue is Epicloe coenophialum, (formerly Neotyphodium coenophialum and Acremonium coenophialum) and it grows within the plant in a mutually beneficial relationship. Through this symbiotic relationship, the plant is aided by increased drought and heat tolerance, improved seed germination and production, seedling vigor, tiller growth rate, improved mineral uptake, and insect and disease resistance. In return, the endophyte receives nutrients, protection within the plant, and dissemination through the seed.
“Fescue toxicosis” refers to the suite of challenges that tall fescue presents to an animal’s health and productivity. Relative to cattle, horses are more sensitive to the alkaloids in tall fescue infected with toxin-producing endophyte. Horses suffering from fescue toxicosis exhibit prolonged gestation, increased incidence of foal and mare mortality resulting from dystocia (difficulty giving birth, or foaling), thickened placenta that have a tendency to separate from the uterine wall prematurely (also known as “red bagging”), agalactia (lack of lactation) in postpartum mares, and less vigorous and immune-challenged foals. This circular provides more information about fescue toxicosis in horses, including management guidelines that can reduce or eliminate the negative effects of the toxic alkaloids.
Signs of Fescue Toxicosis
Prolonged gestation: Normal gestation in mares averages 350 days. Mares consuming endophyte-infected tall fescue are reported to have an extended gestation of two weeks or more past the normal time frame for parturition (foaling, or giving birth). In studies to date, individual mare sensitivity to available ergovaline (ergot alkaloid) levels that fluctuate with environmental factors potentially impact parturition.
Dystocia: The prolonged gestation period appears to contribute to frequent dystocia problems, as well as inadequate preparation of the reproductive tract and fetal malpresentation. Foals tend to have larger than-normal skeletal frames due to the extended gestation period and are often rotated 90 to 180 degrees from the normal position for delivery.
Agalactia: There appears to be a connection between fescue toxicity and milk production by the alkaloid that reduces production of prolactin, which is a hormone released late in gestation that promotes the onset of milk production in the mare. Mares grazing endophyte-infected tall fescue have lower prolactin concentrations throughout gestation and at foaling than those on non-infected pasture.
Thickened placenta: In many cases of fescue toxicosis, foals present normally but are encased in a placenta so tough they cannot break through and suffocate without human assistance. Placentas of mares that grazed endophyte-infected tall fescue are thickened, reddish in color, and heavier than those grazing non-infected pasture.
Foal mortality: Pregnant mares are not the only ones affected by fescue toxicosis—their foals have issues as well. Problems can include still births, weak foals with poor muscle mass, and septicemic foals. Septicemia, a systemic infection from bloodborne bacteria, can partially be explained by agalactia on the part of the mare and foals too weak to nurse. Foals can be large-boned and emaciated in appearance with overgrown hooves.
What Are Your Options?
Pasture Management
There are a variety of methods to deal with the problem of fescue toxicosis in horses. One is to eliminate toxic tall fescue by killing the stand and replacing it with an alternative forage crop. Endophyte-free tall fescue cultivars are available, but they do not persist well in the Southern U.S., nor do they have the drought and pest resistance of the toxic tall fescue varieties. An excellent alternative is to establish winter annual grazing or a novel endophyte infected tall fescue. Novel endophyte tall fescues contain a naturally occurring endophyte that does not produce the alkaloids that are toxic to grazing livestock. Novel endophyte tall fescues have all the positive agronomic aspects of the toxic tall fescue, including persistence, drought tolerance, and pest resistance. For more on novel endophyte tall fescues, see 海角官方首页 Extension Circular 861, Novel-Endophyte Infected Tall Fescue.
However, destroying and replacing a toxic tall fescue pasture or hayfield is inherently risky and quite expensive. The significant cost premium for the seed of the novel endophyte tall fescue varieties makes the renovation even more expensive.
Another option would be to manage the toxicosis by removing late gestation mares from infected tall fescue pastures during the last trimester. Veterinarians recommend removing mares from infected tall fescue pastures 45 to 90 days prior to the expected foaling date. This greatly reduces the risk but does not necessarily eliminate the risk. Since the toxins are sometimes stored in fat cells, some mares ca
